In U.S. Pat. No. 4,276,167, a simplified and efficient structure for effecting diffusion washing, bleaching, and/or thickening or the like has been illustrated and described, and the entire disclosure of said patent is hereby incorporated by reference herein. While the structure as illustrated in said patent has numerous advantages over conventional diffusion washers, and the like, there may be a few minor drawbacks associated therewith, especially for large production vessels.
The structure in U.S. Pat. No. 4,276,167 includes the linear actuator for reciprocating the withdrawal screens, and fluid introduction structures, located at the top of the vessel, with all wash liquid and withdrawal conduits also located at the top. For large production vessels with large and heavy screen structures, the supporting frame for supporting the actuator on the top is inconveniently large and heavy. Further, at start-up one must suck up the liquor being withdrawn, and since the temperature in the vessel is in the range of 70.degree.-100.degree. C., there may be boiling problems.
According to the present invention an improvement over the structure illustrated in U.S. Pat. No. 4,276,167 is provided, which improvement is especially useful for situations where the vessel is a large production vessel. According to the present invention, the linear actuator for reciprocating screens and fluid introducing structures is located at the bottom of the vessel, and the extracted liquor is also withdrawn through the bottom of the vessel. Thus, there is no necessity for a large and cumbersome supporting structure for the linear actuator at the top of the vessel, the withdrawn liquid can be easily passed out the bottom without boiling problems, and the liquid conduit structures in generally can be simpler since the treatment liquor (wash or bleaching) is introduced at either the top or the bottom while the withdrawn liquid is taken out through the bottom.
While the structure according to the invention overcomes potential problems associated with the structure in U.S. Pat. No. 4,276,167, it does so without sacrificing most of the advantages thereof. Although it is necessary to provide a penetration at the bottom wall of the vessel in order to provide liquid withdrawal through the bottom, since the central structure is not rotating, but merely reciprocal, this can be accomplished by a simple packing box, there being no necessity for additional packing boxes, seals, or the like in order to effect a desired action. Thus the packings for the central rotor and for packing and sealing the arm where they extend through the vessel side walls which were inherent in the art prior to the invention in U.S. Pat. No. 4,276,167, still are avoided according to the present invention. A distinct, central rotor can be mounted at the top of the vessel to remove treated pulp.
It is the primary object of the present invention to provide a simple and efficient structure for the washing, bleaching, and/or thickening of pulp or the like. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.